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Full text of "Text Book Of Mechanical Engineering"

Plate Moulding.                              63

•crease in size of pattern necessary to compensate for contraction.
Patterns two to three inches across, or less, should be made
.about By smaller to allow for rapping only, and as this does not
take place in an upward or downward direction, there should
always be full allowance for contraction at these places.

The greatest shrinkage due to cooling will usually occur where
there is the greatest body of metal, and use must be made of
this knowledge by the pattern maker.

The linear contraction for different metals is as follows :—

Cast Iron      ......     |-"    per foot     =     -125"

Brass.........    T»/         „          =    '187*

Gun Metal    ......      -j"         „          =     -166"

Steel.........    fV"         „          =    -187*

Malleable Cast Iron         -f5/         »          =     '^f

Aluminium    ...         ...    \Y         „          =     '265"

Spur wheels about 2 ft. 6 in. diameter, contract B\" per foot,
and such wheels vary their contraction, increasing to •£$" per
foot for a wheel 10 ft. diameter (Box).

Three-foot rules, longer than the ordinary rules by the above
fractions, are called Contraction Rules,' and are used by the
pattern-makers, but with much care and judgment.

When wooden patterns are made, from which are to be
moulded metal ones, a double contraction should be allowed
•on account of the two mouldings necessary to produce the re-
quired casting in the first case, and the consequent double
shrinkage.

Metal Patterns are required for light work or when a great
length of service is required. Such patterns are usually the same
as the wooden ones from which they are made ; but there are
•other examples of moulding with iron or brass patterns, as in
Plate Moulding. This is handy for such small articles as
•occur in a brass foundry; Fig. 730 will shew the method. A
wrought-iron plate a is provided with half patterns on either
side, made in brass and carefully finished. Prints are run for
connecting each pattern, so making channels for the flow of the
metal. The plate also has corners b />, so that when put between
the boxes c c> and rammed up with sand, exact correspondence of
the boxes is obtained. Except for blackening and fixing of